To make the focal point of a light beam follow a target track on a storage layer of an optical disc that is rotating at a predetermined velocity, a tracking error signal representing the magnitude of a tracking error and a focus error signal representing the magnitude of a focus error need to be generated based on the light beam that has been reflected from the optical disc.
Various methods for generating a focus error signal have been developed so far. But to perform a focus control with precision, an accurate focus error signal containing little disturbance signals should be generated. For example, Patent Document No. 1 discloses a technique for minimizing the intrusion of optical modulation components, which would be caused by a guide groove on the track of an optical disc, in a focus error signal.
Specifically, the optical disc drive disclosed in Patent Document No. 1 detects focus errors by generating focus error signals on three light beams that have been focused on an optical disc and subjects these three focus error signals to a predetermined computation, thereby obtaining focus error signals for use to perform a focus control. As a result, a highly accurate signal, in which the optical modulation components have been canceled, can be obtained.
The optical disc drive includes a tracking detecting section of a differential push-pull (DPP) type and a focus detecting section that adopts an astigmatism method. The focus detecting section and the tracking detecting section generate a focus error signal and a tracking error signal, respectively, based on the output signals of a photodetector section.
FIG. 7 shows a configuration for a conventional photodetector section 60, which includes three photodetectors 61, 62 and 63. The quadruple photodetectors 61 and 63 receive two of three light beams at their quadruple areas, while the quadruple photodetector 62 receives the main beam at its quadruple areas. In FIG. 7, the reference signs a through 1 denote the respective photosensitive areas of the quadruple photodetectors. If the signals detected at the respective areas are identified by the reference signs of those areas, the focus error signal FE can be given by the following equation.FE=a−b+c−d+G(e−f+g−h+i−j+k−l)  (1)(G is Predetermined Constant.)
To perform a DPP type tracking detection, the optical pickup is designed such that the phase of a push-pull signal detected from the main beam is different by 180 degrees from that of a push-pull signal detected from the two sub-beams. That is why by multiplying the focus error signal of each beam (e.g., the focus error signal of the main beam is a−b+c−d) by a predetermined constant and then adding the products together as in Equation (1), the push-pull signals (i.e., optical modulation signals caused by the guide groove) can be canceled from the focus error signal FE. As a result, the signal FE can be a highly accurate control signal.
On the other hand, in the optical disc drive disclosed in Patent Document No. 2, the control section monitors the level of the focus error signal FE and starts shifting the focal point provided that the FE signal that has exceeded a first setting is lower than a second setting. As a result, even if the optical disc has significant disc flutter, the focal point can still be shifted steadily to a focus controllable range.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2004-63073 (pages 8 to 9 and FIGS. 2 and 4)        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 8-77569 (pages 3 to 4 and FIGS. 1 and 2)        